1. Field of the Invention
This invention relates to a method of heat treatment of wafers which are formed of a substance of poor heat conduction such as sapphire and spinel. It is chiefly devoted to a method of heat treatment for the oxidation or impurity diffusion of the so-called SOS (Silicon on Sapphire) wafers in which a single crystal silicon layer is formed on the surface of a sapphire substrate.
2. Description of the Prior Art
Likewise to the treatment of semiconductor wafers (for example, Si wafers), the heat treatment of the SOS wafers is conducted under the state under which many wafers are erected at equal intervals on recesses in a wafer jig within a heat treatment furnace. In such method, cracks often arise in the wafers when they are introduced into the heat treatment furnace and drawn out therefrom.
The wafer cracks can be prevented to some extent in such way that the wafers to be processed are slowly introduced into and drawn out from the heat treatment furnace. On the other hand, however, where the heat treatment is made in large amounts, the wafers differ largely in the thermal hysteresis and do not become uniform in characteristics.
In the prior art, therefore, it has been very difficult to prevent the wafer cracks from occurring and to lessen a dispersion in the characteristics of the respective wafers in case where the SOS wafers are heat-treated in mass production.